What are "res" and "req" parameters in Express functions

The "res" parameter in Express functions represents the response object that is sent back to the client. It contains methods and properties that allow the server to send a response to the client, such as sending data, setting headers, and specifying the status code.

The "req" parameter in Express functions represents the request object that is sent by the client to the server. It contains information about the incoming request, such as the URL, HTTP headers, query parameters, and request body.

Within an Express function, "res" and "req" are commonly used as the naming conventions for these parameters, but you can choose any names you prefer, as long as the order is maintained.

For example, in an Express route handler, you can use the "res" parameter to send a response back to the client:

app.get('/users', (req, res) => {
  // retrieve user data from a database
  const users = [
    { id: 1, name: 'John' },
    { id: 2, name: 'Jane' },
  ];

  // send the users as JSON response
  res.json(users);
});

In the above example, when a GET request is made to the '/users' endpoint, the server retrieves user data from a database and sends it back to the client as a JSON response using the "res" parameter.

On the other hand, the "req" parameter can be used to access information about the incoming request:

app.post('/users', (req, res) => {
  const name = req.body.name;
  // store the user in the database

  res.send('User created successfully');
});

In this example, when a POST request is made to the '/users' endpoint, the server can access the name of the user from the request body using the "req" parameter, and then store the user in the database. Finally, a response is sent back to the client using the "res" parameter.